Comparison of Cyclic and Linear Poly(lactide)s Using Small-Angle Neutron Scattering

Small-angle neutron scattering (SANS) experiments were conducted on cyclic and linear polymers of racemic and l-lactides (PLA) with the goal of comparing chain configurations, scaling, and effective polymer–solvent interactions of the two topologies in acetone-d6 and THF-d8. There are limited reports of SANS results on cyclic polymers due to the lack of substantial development in the field until recently. Now that pure, well-defined cyclic polymers are accessible, unanswered questions about their rheology and physical conformations can be better investigated. Previously reported SANS experiments have used cyclic and linear polystyrene samples; therefore, our work allowed for direct comparison using a contrasting (structurally and sterically) polymer. We compared SANS results of cyclic and linear PLA samples with various microstructures and molecular weights at two different temperatures, allowing for comparison with a wide range of variables. The results followed the trends of previous experiments, but much greater differences in the effective polymer–solvent interaction parameters between cyclic and linear forms of PLA were observed, implying that the small form factor and hydrogen bonding in PLA allowed for much more compact conformations in the cyclic form only. Also, the polymer microstructure was found to influence polymer–solvent interaction parameters substantially. These results illustrate how the difference in polymer–solvent interactions between cyclic and linear polymers can vary greatly depending on the polymer in question and the potential of neutron scattering as a tool for identification and characterization of the cyclic topology.

An Agilent 1260 Infinity Multi-Detector GPC was used to gauge relative molecular weights reported in Table 1 (to a poly(styrene) standard) using refractive index methods. For Mark-Houwink plots (i.e. Figure 4), a combination of refractive index, light scattering and viscosity measurements were used to generate the plot of intrinsic viscosity versus absolute molecular weight. Two columns are used to separate polymer chains by molecular weight: a PLgel 5 µm MIXED-D 300 x 7.5 mm, with a guard column PLgel 5 µm MIXED Guard 50 x 7.5 mm. All samples were dissolved in GPC grade THF (2 mg ml -1 ) and left overnight before being passed through a hydrophilic filter. The injected sample volume was 100 µL, the flow rate of the instrument was 1 mL min -1 and the measurements were conducted at 35 ο C.
NMR spectra were acquired on Bruker Avance III 400 and 500 MHz spectrometers Spectra were referenced to solvent peak for CDCl 3 . 2 To determine tacticity of PLA samples, the probability of racemic enchainment (P r ) was calculated using homonuclear decoupling experiments on the methine region of the PLA NMR spectra. Each decoupled peak in this region can be attributed to a specific arrangement of stereocenters according to literature. P r values represent the degree of heterotactic arrangement in the polymer. A value of 1 refers to a fully heterotactic polymer. 3 The DSC model was a TA instruments DSC Q20. DSC measurements of poly(ε-decalactone) samples involved two heating and cooling cycles between -70 and 50 ο C with a heating/cooling rate of 20 ο C min -1 . Reported T g 's in Table 1 were taken from the second curve in the resulting DSC trace, after any thermal history had been cleared.

Polymer Characterization
Cyclic purity was verified by a combination of characterization techniques including: differences in viscosity as seen in Mark-Houwink plots generated from triple-detection GPC (see Figure S2); lack of end groups in both 1 H NMR and MALDI-TOF spectra (see Figures S3, S4, S5 and S6); and increases in glass transition temperatures of cyclic vs. linear polymers (the glass transition temperatures of the cyclic PLA samples were 4-5 ο C higher than comparable linear counterparts -see Table 1 in main text), which is indicative of cyclic topology.
Tacticity was determined using homonuclear decoupling NMR experiments on peaks for the methine region of PLA (see Figure S7). The ratio of peak intensities (which correspond to the difference possible stereosequences of monomers) was used to calculate the probability of racemic enchainment (P r ), giving the degree of heterotacticity between 0 (fully isotactic), 0.5 (fully atactic) and 1 (fully heterotactic). P r values of 0.77 for cyclic sample L4 indicated a strong heterotactic preference in the polymer. This allowed for comparison with the more atactic polymers made using Sn(Oct) 2 /catechol system. 3 Figure S5: MALDI-TOF spectra of a linear PLA sample, showing mostly peaks for linear polymer chains initiated with benzyl alcohol, the co-initiator for this reaction -cyclic species can be seen to be dominant at low molecular weights.
n -2 Figure S6: MALDI-TOF spectra of a cyclic PLA sample, showing peaks corresponding to cyclic species with no end groups and various different counterions. The shoulder peaks (denoted with star symbols) were not observed in other cyclic MALDI-TOF spectra but were assigned as cyclic polymer species. This was due to the well-known effects of linear contaminants on cyclic polymer viscosity, which were not observed in these samples. Cyclic purity was further confirmed in other MALDI-TOF spectra (see Figures S3 and S4) as well as viscosity and T g differences with comparable linear samples.
Additional data from SANS (e.g. Intensity vs. q plots, fit examples, R g data) Figure